Since the emergence of a thermotropic liquid crystal polymer, the heat resistance thereof and the attainability of high strength and high elastic modulus thereby have been noted and thus some prior arts have been developed with respect to the process for producing the fiber (see, for example, U.S. Pat. Nos. 3,975,487, 4,468,364 and 4,161,470 and Japanese Patent Laid-Open Gazette No. 196716/1988).
It is known that the thermotropic liquid crystal polymer can have high strength and high elastic modulus only by spinning if it Is performed under appropriate conditions. Further, it is known that the heat treatment and redrawing can improve the strength and elastic modulus thereof. In particular, it is reported that some types exhibit a strength improvement to 5 to 6 times the original.
In the spinning of the above conventional thermotropic liquid crystal polymer filament, it has been necessary to conduct the spinning with the use of a nozzle having a very minute aperture for obtaining a fine denier filament because it is difficult to increase the draft ratio thereof. Further, extrusion abnormalities such as melt fracture are likely to occur. Thus, the extrusion rate cannot be made high resulting in extremely poor productivity.
Therefore, with respect to the properties of the obtained filament, no processes have been established for stably producing a filament with the ultimately high strength and elastic modulus at high productivity except on a laboratory scale.
The inventors have conducted extensive investigations into the causes of the low productivity of the prior art, the difficulty in stably producing a product with high strength and high elastic modulus and the poor spinning operation efficiency (end breakage, denier nonuniformity and product quality dispersion, etc.) in connection with the melt spinning of the thermotropic liquid crystal polymer filament. As a result, the following has been found.
(1) The thermotropic liquid crystal polymer as a starting material is not always uniform.
The cause is in an aspect an inevitable consequence of the technology of polymerizing the thermotropic liquid crystal polymer. In other aspects, the cause relates to the heat history difference and heat deterioration after polymerization and the increase of polymerization degree by heat treatment. With respect to these, a rapid product quality improvement has been attained for the recent years by virtue of, for example, polymerization and subsequent treatment technologies and post-polymerization filter technologies. However, the improvement is still not satisfactory.
(2) The melting point of the thermotropic liquid crystal polymer is so high that, after leaving the spinning nozzle, the surface of the filament is cooled by the temperature of the atmosphere with the result that the draft ratio cannot be made high.
Thus, a skin layer is formed at the surface of the filament, thereby creating a structural difference between the inner part and the surface part. This is an obstacle of the high quality (for example, high strength, elastic modulus and elongation). In the spinning of the customary thermoplastic polymers, the draft ratio can be increased in the form of a melt having left the nozzle to thereby orient the molecules. However, with respect to the thermotropic liquid crystal polymer, greater belief is in that the orientation is completed in the nozzle, and the thought has not arrived at a concept of stably increasing the draft ratio upon leaving the nozzle.
(3) The nozzle diameter is reduced for increasing the orientation in the nozzle. Further, the reduction of the nozzle diameter is inevitable for obtaining a fine denier because the above draft ratio cannot be increased.
However, the extrusion rate is proportional to fourth power of the nozzle diameter, so that the reduction of the nozzle diameter leads to extremely poor productivity. Further, the shear rate of the extrusion is inversely proportional to third power of the nozzle diameter, so that the reduction of the nozzle diameter leads to extremely high shear rate, thereby causing extrusion abnormalities such as melt fracture. With the use of a polymer having instability factors in its starting material such as the thermotropic liquid crystal polymer, stable operation cannot be conducted at shear rates close to the extremity.
(4) The thermotropic liquid crystal polymer filament is a functional fiber with high strength, elastic modulus, chemical resistance and heat resistance and further excellent electrical properties. However, it is also an industrial fiber, so that how cheaply the thermotropic liquid crystal polymer can be produced is important.
However, because of the above factors (1) to (4) affecting in combination, it has been unfeasible to stably produce a fine denier filament of thermotropic liquid crystal polymer with high strength and high elastic modulus on an industrial scale.
Often, the thermotropic liquid crystal polymer filament is used as an industrial reinforcing fiber in FRP, FRTP, concrete reinforcement and the like. In such uses, the thermotropic liquid crystal polymer must exhibit improved affinity for, adhesion to and uniform miscibility with the matrix.
The above thermotropic liquid crystal polymer has only been formed into fibers but not into a nonwoven web or a filament assembly because special spinning means and subsequent heat treatment are required therefor. In a simple assembly of rigid thermotropic liquid crystal polymer filaments, the filaments cannot be mutually entangled, so that the assembly is readily disintegrated with external force, thereby disenabling the holding of the outline as a filament assembly. Although the fixing with an adhesive can be thought of, the use of the adhesive is unfavorable because it generally degrades the heat resistance and electrical properties of the filament assembly. Further, heat-resistant adhesives are expensive.
A technique comprising shortly cutting the conventionally produced thermotropic liquid crystal polymer filament to thereby form the same into a nonwoven web or a paper has been reported (EPC Patent Laid-Open Gazette No. 167682 (A)). However, not only is the reinforcing effect of short fibers poor in the use in FRP and FRTP but also additional steps such as adhesive bonding and fibrillation for formation into a nonwoven web or a paper are inevitable. The fibrillation has a drawback of degrading the performance of the highly elastic fiber.
The heretofore proposed processes for producing a filament assembly or a nonwoven web from the thermotropic liquid crystal polymer filaments have drawbacks in that not only is a cost increase inevitable but also the production of a nonwoven web or filament assembly of long-fiber filaments is difficult and the quality of the resultant product is poor. Specifically, there are problems such that a binder is requisite for uniformity and filament integration and that the filaments are not loosened.
A filament of high strength and high elastic modulus cannot be obtained by melt spinning the thermotropic liquid crystal polymer through the melt spinning nozzle conventionally employed in the filament spinning followed by free fill and flow. Instead, the filament diameter is unfavorably large and an impracticable filament assembly (nonwoven web) results.
The reason has been revealed to be that the filament having exited the nozzle is oriented by the nozzle shear rate to thereby have increased strength and further the surface of the filament is cooled to solidify because of high melting point, so that only the weight thereof does not lead to application of a draft with the result that the filament diameter cannot be decreased.
An extreme increase In the shear rate in the nozzle for increasing the molecular orientation results in extrusion abnormalities such as melt fracture in the nozzle, end breakage upon exiting the nozzle and block formation (bundling) at the corresponding part in a filament assembly. Thus, a filament assembly of high quality cannot be obtained. Especially, in the filament assembly from the thermotropic liquid crystal polymer, uniform polymer cannot be obtained in the polymerization of the thermotropic liquid crystal polymer as a starting material and, further, thermal polymerization or decomposition is advanced by the influence of heat in the extruder and other means, so that the polymer dispersion is extensive in that some parts of the polymer have extremely high molecular weights or rather in the form of gels while some other parts are decomposed to exhibit low molecular characteristics. Thus, extrusion abnormalities are likely to occur.
Therefore, the current situation has been that a filament of high strength and high elastic modulus cannot be realized in industrially stable conditions.
Moreover, the filaments must be well entangled for forming a filament assembly. A simple assembly of thermotropic liquid crystal polymer filaments which are composed of rigid molecular chains and also thick is readily disintegrated and cannot function as an assembly.
On the other hand, in the use of an assembly of long-fiber filaments of high strength and high elastic modulus in FRP or FRTP, the attainment of uniform mixing of the filaments with a matrix polymer from not only the microscopic but also macroscopic viewpoints encounters an extreme difficulty in practice. Parts where the amount of the reinforcing material is small naturally have less reinforcing effect to thereby cause product defects, while, at parts where the amount of the reinforcing material is too large, not only is this wasteful but also the amount of the matrix resin is unsatisfactory to thereby also occasionally cause defects.
Therefore, in the use of continuous long-fiber filaments as reinforcing fibers in, for example, FRP or FRTP, how uniformly the filaments are mixed with the matrix resin is an important task.
In the above use, it is also important to provide the thermotropic liquid crystal polymer filaments as reinforcing fibers with the affinity for and the adherence to the matrix resin.
In the conventional FRP and FRTP, the arrangement of filaments is random in the filament assembly. In particular, when a plane strength is to be realized by arranging the filaments in a plane form, the current situation is that the filaments are incorporated in the FRP and FRTP in the form of a prepreg or a woven fabric of the reinforcing filaments. In the form of a prepreg or a woven fabric, however, not only are these per se expensive but also the fibers must be used in an excess amount because of a dense structure and, further, multiple layers must be incorporated in the reinforcement of a thick object, so that occasionally a shaped article as a whole becomes too expensive to put to practical use. Moreover, the prepreg and woven fabric may not fit a delicate configuration of a shaped article.
Therefore, a filament assembly is desired which is cheap and soft but has the filaments arranged along an intended direction.
Naturally, how effectively the cost reduction can be achieved is an important task because the thermotropic liquid crystal polymer filaments are mostly used as industrial materials.
U.S. Pat. No. 4,362,777 discloses a nonwoven web of thermotropic liquid crystal polymer filaments. However, the filaments are not entangled because those right under the spinning nozzle are not in a high-temperature atmosphere.
U.S. Pat. Nos. 4,442,266, 4,522,884 and 4,442,057 disclose filaments obtained by spinning of a blend of a thermotropic liquid crystal polymer and, for example, polypropylene. However, the filaments are not entangled because those right under the spinning nozzle are not in a high-temperature atmosphere.